Optical and Chromaticity Properties of Metal-Dielectric Composite-Based Multilayer Thin-Film Structures Prepared by RF Magnetron Sputtering

: Over the last decades, the coated glass products, and especially the low-emissivity coatings have become a common building material used in modern architectural projects. More recently, these material systems became common in specialized glazing systems featuring solar energy harvesting. Apart from achieving the stability of optical parameters in multilayer coatings, it is also important to have improved control over the design of visual color properties of the coated glass. We prepare metal-dielectric composite (MDC) based multilayer thin-film structures using the RF-magnetron sputtering deposition and report on their optical and chromaticity properties in comparison with these obtained using pure metal-based Dielectric/Metal/Dielectric (DMD) trilayer structures of similar compositions. Experimentally achieved Hunter L, a, b values of MDC-based multilayer building blocks of coatings provide a new outlook on the engineering of future-generation optical coatings with better color consistency and developing approaches to broaden the range of achievable color coordinates and better environmental stability.


Introduction
The prime cause of global warming is the continued use of fossil fuels. The excessive use of fossil fuels could lead to the non-renewable energy reserves being depleted, and that demands exploring novel materials and alternative technologies to minimise energy consumption. Besides the traditional industrial ways of "green" energy harvesting, the development of energy-efficient-buildings, building-integrated photovoltaic (BIPV) systems, and window coatings can represent the most sustainable approach to building a pathway towards a fossil-fuel-independent future and decreasing the CO2 footprint [1][2][3][4][5]. Modern integrated PV technologies consisting of specialised thin-film coated glass, known as low-emissivity (Low-E) coatings, can cut either the heating-related, or the coolingrelated (or in some cases, both) electric energy usage in buildings. The Low-E coatings are typically structured as metal-dielectric multilayer thin films, which are designed to prevent heat leakage (from the inside of buildings into the environment) by providing strong and very wide-band reflection of thermal infrared wavelengths. Silver-based metal-dielectric multilayer thin-film coatings are very common currently in building windows, which can also offer maximised "light-to-solar gain" -the ratio between the visible light transmittance and solar heat gain coefficient (SHGC).
In recent years, the optical properties of metal-dielectric coatings and their influence on building energy efficiency have been studied extensively due to their superior performance compared to other multilayer coating structure types. The metal-dielectric coatings performance mainly depends on the choice of materials, and its design philosophy [6][7][8][9][10][11][12][13][14]. Most (or practically all) products available today from this category (featuring high spectral selectivity and high LSG) employ at least two silver (Ag) layers within their structure, and these coatings cannot withstand prolonged (weeks-scale) exposure to either the ambient atmospheric air or dry-heat test temperatures in excess of 160-180 °C. However, the stability problems of ultrathin Ag layers (normally of less than 20 nm thickness) originate from a range of factors related to the metal layer growth morphology. To overcome these issues, a significant number of research works have been proposed, conducted, and resulted in reporting various solutions, where some of them require more complex materials processing and process parameters optimization [9][10][11][12][13][14][15][16][17][18]. Several reports have been published about the properties of various MDC thinfilm layer chemistries, including Ag+SiO2, Ag+TiO2, and Ag+MgF2, however, most of them were about metal-dielectric nanoparticle array properties of interest for various applications [19][20][21][22]. We previously proposed and partially implemented the development of optimized metal-based RF magnetron co-sputtered MDC (e.g. Ag+ MgF2) layers to replace the pure metal layer in those optical coatings [23,24]. To the best of our knowledge, the majority of published literature describing Ag-MgF2 and similar nano-composites research relates to applications other than the product-level development of advanced transparent heat regulating (THR) coatings for use in modern construction. We practically prepared and demonstrated MDC layer-based optical coating building blocks with high structural and optical durability and stability [23,24], however, until recently we have not explored the color chromaticity properties of those multilayer coating types.
Metal-dielectric thin-film coatings (spectral filters featuring controlled apparent colors) have numerous application possibilities in the fields of building architecture, windows, printed products, and other decorative coatings. There are several reports which have been published in the literature about the color properties of different types of thin-film materials (most of these are single-layer), except a couple of them were found to deal with the color properties of multilayer thin-film structures [25][26][27][28][29][30][31][32][33][34][35]. For example, Dalapati et. al., reported about the preparation and properties of color-tunable low-cost transparent heat reflectors using copper and titanium oxide for energy-saving applications [36]. However, they proposed to control the color properties of the multilayer structures by means of post-deposition annealing process, which can force the metal layer to transform into an array of nanoparticles and can change the optical properties e.g. the transmission spectra. Hence, the color stability of property-optimized application-specific metal-dielectric thin-film coatings and their easy manufacturing process for industrial practices are yet to be achieved for future commercial applications. In this work, we focus on establishing a reliable fabrication process of MDC layers, which can open the new possibilities in engineering and development of new DMD-type coatings with highly durable and reproducible optical and color properties.

Growth and characteristics of thin metallic films, metal-dielectric composites, and multilayer structures
We deposited several batches of thin metallic layers, metal-dielectric composites, and multilayer DMD-type building blocks on Corning Eagle XG glass substrates by using the experimentallydetermined, fine-tuned process parameters in RF magnetron sputtering systems. In order to achieve very accurate film thickness control for each type of films (either the single layers, or multilayer structures), all the materials/processes used in this work were well calibrated by way of running multiple characterization experiments (described in the Supplementary section). It is very important to have a very precise and controlled growth process for thin metallic films, to enable developing application-specific thin film-based devices and optical components. For example, in DMD-type multilayer structures, thin metallic films play an important role in achieving the required optical transparency, reflectivity and the color of the coatings. It is well known that the deposition of highquality thin metallic (Ag) layers (of less than 20 nm thickness) requires multi-disciplinary work and research, and involves using technical insights from the fields, such as materials chemistry, nanoscale characterization, and vacuum technology. In the published literature, a claim for the successful fabrication of an ultra-smooth (and ultra-thin, at around 10 nm) Ag layer by using DC magnetron sputtering system is reported in [37], however, rather than using conventional glass substrates, a specially prepared InP substrate was needed to grow this ultra-thin Ag layer. We made multiple deposition runs to re-confirm the suitability of all the selected process parameters as well as the tooling factor (TF %, a crucial parameter) related to the thin-film growth. Thin Ag layer thickness was monitored during the deposition process with the help of quartz microbalance sensor. Also, from the broad-bandwidth spectral fitting of the measured transmission spectra of Ag (single layer) films with the modelled transmission spectra, the film thicknesses were reconfirmed and then the final tooling factor for the pure metallic targets was recalculated, until stable values were identified. The details of the fine-tuning of this calibration process for thin Ag layer growth are documented in the Supplementary section.
After the successful process development for thin Ag layers, we prepared several batches of metal-dielectric composite (single-layer) films on glass substrates using two separate sputtering targets (Ag and MgF2), to establish the optimized process parameters to deposit MDC layers. The volumetric content of MgF2 addition to the pure-metal layer was calculated by using the separately measured partial deposition rates of the sputtering material targets used (calculation details are provided in the Supplementary section). The volumetric content of MgF2 addition was controlled by the externally applied RF power densities to the both the metallic and dielectric target materials. Figure 1 presents the co-sputtering geometry of the growth of metal-dielectric (Ag/MgF2) nanocomposites. Later, several batches of DMD-type multilayer coating building blocks using pure silver (Ag) layers, as well as the MDC layers, were fabricated on clear glass substrates. Note that the multilayer structures (D/Ag/D and D/MDC/D types) were deposited in a single deposition run by sequential sputtering of layers using Ag, MgF2 and ceramic Al2O3 sputtering targets with material purity over 99.99%. The process parameters and conditions used to deposit pure Ag, Ag-MgF2 (single layer) thinfilms, and the multilayer structures are summarized in Table 1. Table 1. Summary of sputtering process parameters and conditions that were used to deposit single-layer thin-films and multilayer coating building blocks. The optical transmission and color chromaticity properties of single and multilayer films were characterized by using Agilent Cary 5000 spectrophotometer and Konica Minolta 508D colorimeter.

Process Parameters
The chromaticity values of ultra-thin metallic, MDCs and multilayer structures were characterized based on the Hunter L, a, b color scale. Hunter L, a, b color scale is mainly based on the Opponent-Color Theory which assumes that the receptors in the human eye perceive pairs of opposite colors such as light vs. dark, red vs. green, and yellow vs. blue. The L value indicates the level of light or darkness (meaning white or black perceived color) of the sample, while a and b values represent the redness or greenness, and the yellowness or blueness, respectively. Therefore, the explanation of opponent color scale L, a, b stands as follows:

Optical and chromaticity properties of single-layer ultrathin metal layers
The spectrally fitted transmission spectra of the as-deposited Ag thin layer with the corresponding modelled transmission spectra (as shown in Figure 3) confirm not only the film thickness but also the successful optimization of thin Ag layer deposition process parameters. More measured transmission spectra (also fitted with the modelled transmission spectra) are provided in the Supplementary section, with the details of optimizing the materials' tooling factors to deposit ultra-thin metallic layers with minimum film thickness errors. Figure 3(a) represents the transmission spectrum of a 17.5 nm as-deposited Ag layer with its modelled (best fitted to 18.2 nm) spectra. From this thickness error, the material's tooling factor was recalculated, and we made multiple deposition runs to finalize the material's tooling factor, as can be confirmed by the very close matching transmission spectra of the ultra-thin 12.6 nm Ag layer, shown in Figure 3 Table  2. For all single-layer thin Ag films, we focused on the L values mostly, rather than a and b values, as the apparent films' (single layers Ag, or an Ag-dominant nanocomposite) color will mostly be determined by the color (either bright white or transmissive white) that can evaluated by the human eye. The measured L values of the as-deposited films were found to be closer to the simulated L values (with an accuracy of about 2 %). The reason for having slightly lower L values in Ag thin-films, could be due to the thin metallic surface oxidation (films were exposed to room air after the deposition and measurement process, therefore a native oxide could form quickly), however, the measured a and b values for all Ag samples were found to be positive, in agreement with the simulated values. It can be noticed that the L values (both in simulation and measurements) increased with the increasing layer thickness, thus confirming the whitish color of silver layers.  Table 2. Chromaticity characterization results (simulated using OptiLayer Pro software, and measured using Konica Minolta colorimeter, for the light reflected off the coating layers).
The measured optical and chromaticity properties match and fitted well with the simulated values for the single-layer films, confirming the successful deposition and fine control of process parameters used during the deposition of ultrathin Ag layers.

Optical and chromaticity properties of single-layer ultrathin metal-dielectric composites
Several batches of MDC layers were deposited on glass substrates and the layer thicknesses (during the deposition process) were in-situ controlled by measuring the deposition run time. However, the actual (physical) MDC layer thicknesses were later re-confirmed using the transmission spectrum fitting (comparing the measured and modelled transmission spectra). The transmission spectra of measured MDC layers were fitted (across wide bandwidth) with the modelled nanocomposite layer's transmission spectra, to confirm the actual thickness of the asdeposited MDC layer. Also, the transmission spectra of these MDC layers were fitted and compared with the optically-equivalent (modelled) pure-Ag layers. The modelling of the MDC layers was performed by using a software package (OptiLayer Pro), where the refractive index (n) and extinction coefficient (k) values for the MDC layers were generated by using the weighted-average values of the known dielectric permittivity spectra of both component materials (Ag and MgF2) . Figure 4 presents examples of MDC layer physical thickness determination through the optical transmission spectra fittings with the modelled transmission spectra. Figure 4 also shows the calculated absorption coefficients of the MDC layers having different volumetric content of MgF2 dilution into the metal-dielectric composite matrix. The measured transmission spectra of MDC layers were used to calculate the absorption coefficient spectra of the as-deposited MDC layers by using a simple formula, defined as T= exp [-α(λ)d], where α is the absorption coefficient, T is the transmittance (in percentage), and d is the thickness of the films [43,44]. The absorption coefficient of the MDC layers were found to be very similar to that of pure-Ag layer. The absorption coefficient of the MDC layers are presented with their calculated upper and lower limits (considering having up to about 5% error in the final film-layer thickness determination). All of the as-deposited MDC layers showed similar trends of the wavelength and film-thickness dependent absorption coefficients. More transmission fitting and absorption coefficient calculation results are provided in the Supplementary section. 10 nm (Model-fitted 9.5 nm) 54. 16 Figure 5 represents the fitted MDC (Ag+ 5 vol. % MgF2) layer thicknesses (brown-colored dotpoints) and the thicknesses optically-fitted to the optically-equivalent pure-Ag layers (green-colored diamond-points) data-points, plotted against the "apparent sum" thickness of the nanocomposite layer (as would be determined, in a simplified way, by the simple multiplication of the algebraic sum of the partial deposition rates for Ag and MgF2 and the process duration times). A linear trend of increasing MDC layer thickness, as well as the optically-equivalent Ag layer thickness has been observed with the increasing deposition time. However, the actual MDC layer thicknesses were found to be lower than the simple numerically-calculated "apparent sum" thickness. For example, if Ag was deposited at 5.7 nm per minute and an additional 0.3 nm/min rate applied for the MgF2 deposition for the same process duration, the discrepancy between the "apparent sum" and the actual fitted thickness can then suggest that some degree of inter-solubility, or the "compaction" of materials has been occurring during the nanocomposite formation process. It is also possible that the MgF2 molecules may position themselves in-between the nanoisland-or nanocluster-shaped thin metal volumes, and this could help grow comparatively smoother, dielectric diluted metal-dominant nanocomposite layers. We have noted in our MDC-based Low-E film characterization experiments that an improved correspondence between the spectral design specifications and the measured film spectra were seen frequently, compared to using pure-metal layers; this is the reason we suggest that dielectric dilution could improve interlayer interface quality.
Note that the color chromaticity values (Hunter L, a, b) of the MDC layers were also found to be very close to those of optically-fitted equivalent thin Ag layers. The obtained Hunter L, a, b values (both measured and simulated) of all the as-deposited MDC layers (Ag+ 5-10 vol. %MgF2) are summarized in Table 3. The measured L values for all of the as-deposited MDC layers were found to be close to the simulated L values.  We have noticed that the volumetric fraction of the additional MgF2 introduced into the composite system had also a significant influence on the color values. An estimated 22 nm MDC layer having 10 vol. % of additional MgF2 content showed a much lower L values compared to the MDC layer with 5 vol. % (of nearly the same film thickness 22.6 nm), while the a and b values remained almost the same.

Metal layer based DMD type multilayer structures
DMD-type thin-film multilayers are a widespread and active area of current research in the field of thin-film nanomaterials due to multiple possible applications in optics, photonics, plasmonics, energy generation, and opto-electronics. Despite the relatively extensive history of these studies, especially the silver-based DMD multilayers, these are considered as the prime candidates to be used in heat-mirror applications for many years. This is due to their suitability for adjusting the spectral selectivity (high transmission in the visible spectrum and high reflection in the IR region). We opted to prepare a simple DMD-type structure Al2O3 / Ag / Al2O3 to use as a reference guide to investigate and compare the features, properties, and the feasibility of the metal-dielectric nanocomposite layers and their integration into the DMD-type multilayer structures. We prepared pure-Ag layerbased trilayer structure (where a 15 nm Ag layer was sandwiched in between two Al2O3 layers of the same thickness) using the optimized process parameters for each material system. The multilayer structures were deposited on optically clear glass substrates using a single sequential deposition run in an RF magnetron sputtering system. Figure 6 shows the optical and color chromaticity properties (both measured and simulated) of the trilayer thin-film structures. The fitted optical transmission spectra of the trilayer structure (shown in Fig. 6a) confirm the successful deposition of DMD type multilayer structure with high accuracy in layer thicknesses. It can be noted that the middle thin metal layer was closely fitted with 16 nm, whilst we deposited 15 nm in the trilayer structure. The measured Hunter L, a, b values (Fig. 6b) of the asdeposited trilayer building block were found to be quite close to the simulated values for the same trilayer structure. However, we noticed that the color values of pure Ag layerbased trilayer structures changed (as expected due to high oxidation rate of the pure metal layer) drastically within a couple of days of air exposure in lab conditions (detailed results are presented in Section 2.4). It is important to achieve the structural coatings color with good colorimetric accuracy and durability by way of using repeatable and reliable production methods. Metaldominant nanocomposites can represent a suitable alternative to replace the pure metal layers in DMD-type multilayer structures and to engineer highly durable application-specific structural coatings with the desired optical and color chromaticity properties. We prepared several batches of MDC-layer based DMD-type multilayer building blocks to investigate the possibility of integration of MDC layers into more complex multilayer coating systems. The optical and color chromaticity properties of MDC-based DMD building blocks are detailed in Section 2.4.

MDC layer based DMD-type multilayer structures
Al2O3 / MDC (Ag + 5 vol. % MgF2) / Al2O3 trilayer building blocks were deposited onto glass substrates where the nanocomposite layer was sandwiched in-between two metal-oxide layers of same type and thickness. Figure 7 shows an example of physical appearance and the optical transmission spectrum obtained from a MDC layer-based trilayer structure. It can be seen that the apparent color of the MDC based structure is quite similar to that of the pure Ag layer based structure (Fig. 7a, b). The obtained transmission spectrum of the MDC-based structures confirms the possibility of engineering and producing new optical coatings, filters or redesigning and modifying the existing coatings by using metal-dominant composite layers, which can then offer better environmental stability properties compared to similar structures based on pure Ag layers. This can be tested and confirmed by running dry-heat exposure tests in air atmosphere; some of the relevant results were previously reported by our group [23,24,45]. The measured and simulated color chromaticity (Hunter L, a, b) values for several MDC-based D/MDC/D type structures are summarized in Table 4.   Figure 8(b). It can be seen that the L value of the pure Ag layer-based structure has changed drastically within the first seven days after the deposition, while the MDC-based coating structure showed a minimal change in the measured L values. We have also noted that the Hunter a and b values in both trilayer types did not show significant changes on the same time-scale. The changes in L values (negative trend) observed in the pure metal-based trilayer structure suggests that the Ag layer degradation (due to possible silver oxidation) occurred within the DMD structure, and this likely changed the overall film color properties, which manifested in the observed L value changes. On the other hand, the MDC based coating structure showed an almost unchanged L value (a slightly positive change in L has been noted) during up to 15 days of testing.

Summary
The design variations, material inter-compatibility issues, and the optical and chromaticity properties of the MDC-based coatings have been investigated and compared to those of pure metallic layer-based trilayer building blocks of low-emissivity coatings. The study has focused on the optical performance and environmental stability characteristics of silver-containing thin and ultrathin coating structures. Magnesium fluoride-containing MDC layers can be used to enable arbitrarily complex high-stability low-E, heat-mirror, or spectral filter-type optical coating designs of high spectral selectivity, on a range of optical substrates. In terms of color stability, it has been confirmed that the MDC layers can provide more design flexibility in structural optical coatings with the potentially improved tuneability of the apparent color essential for various modern industrial and commercial applications.
The co-deposited metal-dielectric nanocomposites (MDC), in which the volumetric content of the dielectric phase were within 5-10 vol. % are semi-metallic substances (often termed cermets in the literature) of solid solution type that can represent an alternative coating design option, compared to using pure metallic layers. The development of MDCs of composition type Ag:MgF2 was expected to accomplish the following: (i) promote adhesion of the metallic layers to their substrates, (ii) promote mechanical hardness of soft plasmonic metal layers, (iii) improve metal layer morphology by way of "filling the pores" between ultrathin metal nano-islands, and (iv) potentially smoothen the interface roughness in ultrathin metallic layers. The preliminary experimental results obtained in this work confirmed that these types of nanocomposite type metalrich materials will enable the design and fabrication of advanced new types of multilayer optical coatings, even though a number of further studies of these nanocomposite materials are still required. Additionally, it is possible to engineer a broader range of reflected or transmitted color properties, compared to pure metal-based designs.

Conclusions
We have established the RF sputtering deposition process of metal-dielectric composite layers and developed MDC layer-based trilayer structures of interest for use as building blocks in future low-emissivity coatings. The possibility of engineering multilayer optical coatings with improved stability of the transmission spectrum and apparent color properties has been experimentally demonstrated. Dielectric/MDC/Dielectric type trilayer structures provide new options in the design and engineering of next-generation optical coatings enabling new applications in building energy efficiency, architectural glass coatings, heat-shields, and decorative coating products.